1. Field of the Invention
This invention relates to a method for cleaning articles by vapor degreasing; and more particularly to a method of removing organic materials from metallic and electrical materials with a solvent employing various blends including dibromomethane and appropriate stabilizers.
2. Background
The utilization of vapor degreasing techniques has found wide acceptance in industry in the past. This method of cleaning typically involves the heating of a solvent to a boil and the generation of a vapor layer into which the object to be cleaned is placed. This vapor layer is a mixture of air and solvent, where the air has reached the saturation point with the solvent. The vapor is generated by boiling the liquid. It condenses upon the object placed in the vapor layer and drips off of the object into a holding tank. This condensation dissolves the hydrocarbon contaminants and removes it from the object, thereby cleaning it.
This technique is a preferred method of cleaning precision parts, such as electronics, machined metallic parts, etc., since vapor cleaning leaves virtually no residue upon the parts.
Degreasing has been found particularly valuable for the removal of flux from soldered circuit boards. A typical circuit board consists of a thin plate normally of epoxy resin or similar material reinforced with glass fibers carrying electrical connectors on one or both sides consisting of thin strips of copper or other electrically conductive material. Electronic components are normally placed on the side opposite the conductors and by means of leads passing through holes in the board are attached to the connectors by crimping the leads followed by soldering. Soldering is typically carried out by first coating the connector side of the board with a flux and then passing the side of the board over a surface of molten solder. The fluxes employed in the use consist for the most part of rosin. Increasing popular in the trades are rosin fluxes which are activated by the addition of ionic materials. Such activators insure better solder bonds, especially on slightly corroded connectors and leads.
A choice of solvent for removal of the flux is restricted by the insuring; that the solvent will not attack the board or the various material of construction or any of the electronic components contained thereon. Alcohols have been used in the past, however, their use is limited because of the severe danger of fire. Non-flammable materials employed in the past as a solvent medium include CFC 113, 1,1,1 Trichloroethane, perchloroethylene, 1,1,1 Trichloroethylene and various hydrochlorofluorocarbons, such as "Gensolve" (manufactured by Allied Chemical).
Such vapor degreasing technique employing same or similar materials is taught by U.S. Pat. No. 3,881,949 which issued on May 6, 1975 to Carl Martin Brock.
The solvents described above and in the Brock patent are typically on the Clean Air Act list of ozone depleting chemicals and are being phased out of production. It is obvious from the foregoing that a suitable replacement is necessary to try to fill in for these banned ozone depleting chemicals.
U.S. Pat. No. 4,056,403 issued to Robert J. Cramer et al on Nov. 1, 1977 describes a method in which a number of non ozone depleting chemicals, including methylene bromide or dibromomethane are used in cleaning polyurethane foam generating equipment. Cramer et al teach a method wherein a solvent composition described therein is used for cleaning polyurethane foam generating apparatus or a segment thereof and noting particularly the cleaning operation where rinsing or flushing the entire foaming apparatus takes place after the foam forming chemicals have been exhausted. The solvents taught may be periodically injected under pressure through the mixer portion of the foaming apparatus in order to purge it of residual unreacted or partially foam forming materials. The method described in this patent would be totally ineffective because its composition does not include the appropriate stabilizers which would prevent the dibromomethane from becoming acid and thereby attacking the metal surfaces which might be placed into the vapor layer. Use of hot saturated vapors of a liquid halogenated hydrocarbon, including dibromomethane is taught in U.S. Pat. No. 4,193,838 which issued to Robert J. Kelly et al on Mar. 18, 1980. This patent teaches the maintenance of a pool of hot saturated vapors of a halogenated acyclic hydrocarbon then placing pieces of coated paper stock, which had been coated with "hot melt" coatings such as are used on consumer items and milk cartons, etc. These pieces of paper stock are placed in the vapor pool after which they are agitated. Again it is noted that this method would be ineffective at cleaning circuit boards and other metallic materials because of the acidic nature of the non stabilized compound utilized therein which would tend to destroy the object rather than just clean it.
Obviously it is the object of the present invention to provide materials which are suitable for vapor cleansing of electronic and metallic parts without the use of ozone depleting chemicals with a material that would not be appropriate for the cleaning of metal.